In order to treat cancer, it may be necessary to remove or destroy tumourous tissue in a body of a patient. Due to advances in imaging technology and the availability of highly controllable medical devices, ablation technology is becoming a viable treatment option for a variety of non-resectable tumours.
Currently, three ablation methods are commonly used:                RFA (radio frequency ablation) is the most commonly used ablation method that kills tumourous tissue by converting radio frequency energy into heat. RFA devices typically come with diameters of 1.2 to 1.6 mm and may produce ablation zones with a diameter of 1.5-7 cm.        Another widely available ablation therapy method is cryoablation, where instead of heating the tissue, freezing is used to cause necrosis of tissue. In this method, cell death is due to rapid freezing of intercellular water. To freeze the tumourous tissue, special needles may be placed percutaneous, i.e. through the skin, into the tumour. Argon gas is delivered through the needle thereby producing an iced ablation volume of predictable size and shape. Cryoablation devices typically come with diameters of 1.2-8 mm and may produce ablation zones having a diameter of 4-5 cm.        Microwave ablation is an upcoming thermal ablation technology in which tissue may be heated using microwaves. Microwave ablation probes typically come with diameters of 1.2-5.7 mm and produce ablation zones having a diameter of 1.7-6 cm.        
The predicted ablation volume within which a specific ablation device is intended to ablate/kill tumourous tissue, are usually determined and supplied by the device manufacturers. For example, with cryoablation devices, isotherms are generally specified at three different temperatures as shown in FIG. 1. Therein, various volume regions 3a, 3b, 3c may be defined within an overall ablation volume 1 generated by a cryoablation needle 5 such that the surface of each volume region 3a, 3b, 3c is defined at a location having a respective temperature such as for example −40°, −20° and 0°. Accordingly, a volume inside an isotherm has a temperature below the temperature value of the isotherm. Thus, the 0°-isotherm defines the overall ablation volume 1 within which tissue is iced during cryoablation. As the ablation needle 5 forms a symmetry axis during a cooling process, each of the ablation regions 3a, 3b, 3c as well as the overall ablation volume 1 corresponding to the 0° C.-ablation region 3c may have an ellipsoid or spheroid shape. For example, the spheroid enclosing the highest temperature volume region 3c may have main axis having dimensions of 32 mm×56 mm whereas the inner volume region 3a enclosed by the −40° C.-isotherm may have dimensions of 14.5×34 mm. An ice front may extend about 6 mm in front of a needle tip.
During an ablation procedure, a patient is usually positioned in a CT or MRI scanner and one or more ablation needles are inserted into the tumourous tissue under guidance of real-time CT fluoroscopy or ultrasound. The procedure is most often performed by an interventional radiologist in a radiology department of a hospital.
Therein, some of the challenges for achieving of successful necrosis typically are:                a treatment of intermediate and large tumours having for example dimensions larger than the typical dimensions of an ablation volume of an ablation needle;        a precise and reproducible ablation needle placement, i.e. an insertion of the ablation needle into the patient's body towards a correct location within the tumourous tissue such that a location and an orientation of the ablation needle is chosen such that during an ablation process all of the tumourous tissue is destroyed.        
A preferred method of creating a large and/or complex ablation volume is to use a plurality of ablation needles with multiple overlapping ablation volumes. Conventionally, such overlapping ablation volumes may be mostly created in the mind of an interventional radiologist without visual feedback. Due to the fact that, for the example of cryoablation, ablation isotherms are usually asymmetric sized, i.e. may have an ellipsoid shape, depending on a needle insertion direction, an effected area may be determined by both an ablation needle target location and an ablation needle insertion direction/orientation. Thus, it may be clear that precise control of an effected ablation volume may be difficult without precise and reliable assistance by a user such as a surgeon or a radiologist, especially at oblique insertion angles of the ablation needle.